There is continuing demand for manufacturing processes that may lessen the cost, time or steps in producing a part. More often than not, the benefits resultantly associated with improving the manufacturing process are necessitated in the first instance by customer requirements to develop and improve products to have superior dimensional, mechanical and/or performance properties.
For instance, a typical bevel gear such as a differential side gear used for torque transfer and reduction at the differential end of a vehicle wheel axle or drive shaft may have any or all of the following performance requirements: the spline area requires dimensional precision, high shear strength and brinnelling resistance; the hub and thrust faces requires dimensional precision, surface finish and case compatibility; the gear geometry requires dimensional precision, surface finish and optimised profile; and the tooth and core strength may require impact resistance, wear resistance, spalling resistance, and different surface and core metallurgies.
Referring to FIG. 8, in order to meet some of these performance requirements, a gear 310 has typically been made by forging a powder metal 314 and then case carburizing the gear 310 to achieve a nearly constant effective case depth 316. The case depth 316 for each gear tooth 312 is shown in the partial cross-sectional view of FIG. 8. However, a case carburized gear does not necessarily achieve the desired mechanical properties such as enhanced tooth wear and fatigue strength while providing beneficial performance characteristics in the body of the gear.
As another example, a constant velocity joint (CVJ) is a torque transfer shaft coupling that is widely used in vehicle wheel drive shafts to transmit power through a variable angle from a first rotating shaft to a second rotating shaft. One type of CVJ includes (1) an inner race having an inner spline and an outer surface with ball tracks, (2) an outer joint part also having ball tracks, and (3) bearing balls located between the ball tracks of the inner race and outer joint part.
With reference to FIG. 1, a conventional CVJ inner race 10 is made by cold forming, forging, and then case carburizing a metal 12 to achieve a nearly uniform effective case depth 14. The case depth 14 for each of the ball tracks 16 and the splines 18 are indicated by the dashed lines in the partial cross-sectional view of FIG. 1. The processing parameters to achieve a nearly uniform carburization of a fully dense part of a specific hardness, case depth and carbon gradient are generally known. Secondary operations, such as machining and grading, may be performed to ensure dimensional accuracy of the various features of the inner race 10.
Unfortunately, a nearly uniform case depth does not necessarily achieve the desired mechanical properties for a CVJ inner race or a bevel gear. Although a relatively deep hardened area in the ball tracks or gear teeth is beneficial to provide a robust load bearing zone and prevent wear, spalling, or brinnelling during use, it is less desirable to have a hardened spline area. If the area around the splines is hardened, then it will be difficult to machine the splines in a cost-effective way to obtain the required dimensional tolerances.
Therefore, there is a need for an improved powder metal parts. In particular, there is a need for torque transfer powder metal parts used in vehicle wheel drive lines that have different properties in different regions of the part.